Preform for Optical Element and Optical Element

ABSTRACT

A preform for an optical element is provided which involves less amount of deformation of glass in molding, and readily improves lifetime of the mold. The present preform for molding an optical glass element  10  exhibits an almost circular shape having a predetermined diameter in top view, exhibits a flattened semicircular shape having a predetermined overall height in side view, has a concave face on the top surface such that the predetermined overall height is attained at approximately the central position of the aforementioned circular shape, and has a concave face so as to fit along the convex face of the top surface such that a space is provided on the bottom face. The under surface may be either a concave face or a convex face. Also, the top surface may be either a concave face or a convex face.

This application is based on and claims the benefit of priority fromJapanese Patent Application No. 2004-359591, filed on 13 Dec. 2004,Japanese Patent Application No. 2005-047276, filed on 23 Feb. 2005, andJapanese Patent Application No. 2005-187810, filed on 28 Jun. 2005, thecontent of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a preform for an optical element formolding optical elements such as lenses and the like used in opticalequipment, and to a method for manufacturing the same.

BACKGROUND ART

In recent years, optical lenses molded to have a predetermined shapehave been used as lenses for digital cameras and the like, and concavelenses as well as biconcave lenses have become common opticalcomponents. In light of precise and large-scale manufacture of theseoptical lenses, methods in which a preliminary molding is produced firstby molding a molten glass material to have a shape as approximate aspossible to the shape of the final optical element, and the preliminarymolding is supplied to a final mold and subjected to hot working havebeen generally employed.

According to this method, reduction in amount of deformation of theoptical glass in hot working, and shortening of contact time with themold may be achieved, thereby resulting in decrease in defective moldingand prolongation of lifetime of the mold. In addition, effects ofshortening of molding tact time may be expected.

Thus, methods of manufacturing an optical element in which preliminarymolding is conducted once have been disclosed in various documents.However, there do not exist many which refer to the shape of thepreliminary molding.

On the other hand, a glass preliminary molding having a particular shapehas been disclosed (for example, Japanese Patent Application FirstPublication No. H05-213622). In this document, a glass member previouslycut to have a predetermined size is placed on a supporting member havinga circular opening, and is once heated for softening to approximate tothe catenary curve. Disclosed is a method of obtaining a glasspreliminary molding having a biconcave face by further placing thisglass member approximated to the catenary curve in a mold with the upperand lower mold halves having a convex surface, followed by carrying outpress molding.

However, because this glass member approximated to the catenary curvehas a convex shape on one face, it is unsteady in precise press molding.Therefore, defective molding is liable to occur. Furthermore, there is amethod in which a glass material is cut away from a glass blockmaterial, and is then ground and polished to give an optical elementhaving a biconcave face. However, in this case, a long time may berequired for carrying out many steps in an alternating succession, whichmay consequently result in increase in cost.

Meanwhile, a glass preliminary molding previously having aconvexo-concave or biconvex shape was disclosed (for example, JapanesePatent Application First Publication No. H09-12318).

However, the glass preliminary molding described in the document isbased on requirement of change of the mold for preliminary molding toappropriately correspond to the final shape of the optical element, soto speak; therefore, a certain glass preliminary molding cannot adapt tothe pressing for every final shape. Therefore, consideration of thelifetimes of molds for manufacturing the preliminary molding has becomenecessary, and thus the method is far from being envisaged as anultimate solution in light of reduction in cost.

DISCLOSURE OF THE INVENTION

The present invention provides a preform for an optical element whichenables prolongation of lifetimes of molds in a series of steps ofmanufacturing an optical glass element and enables reduction in percentof generation of defective molding articles and which can be producedaccording to a simple method, and provides a method of manufacturing thesame.

A first preform for molding an optical glass element of the presentinvention has a somewhat flattened hemispherical shape, and its halvedface includes a concave face. Moreover, a second preform for molding anoptical glass element of the present invention is a preform for moldingan optical glass element having a somewhat thick discoidal shape, whilethe top surface and the under surface both having a concave face. Also,a third preform for molding an optical glass element of the presentinvention is a preform for molding an optical glass element having athin flattened shape, while the top surface and the under surface bothhaving a convex face.

According to these embodiments, it is expected that upon pressing with aconvex pressing die or a concave pressing die in the following moldingstep, the aforementioned concave face or convex face may match thisconvex pressing die or concave pressing die, or fit thereto, therebyallowing the pressing force of the pressing die to be uniformly appliedto the preform. Furthermore, the preform does not usually have aprojection or the like that engages with another member or component,and its settling position may be mostly determined by gravitation andshearing force resulting from compressive force, frictional force andthe like applied from the surface to be brought into contact.Furthermore, how it is settled may decide whether it is stable (stableagainst some variation) or unstable (unstable because some variationcauses further greater variation). It is likely to be unstable when theshape of the pressing die is different from the shape of the face of thepreform to be in contact. Therefore, a preferable shape of the preformand contact with the pressing die will allow the pressing force of thepressing die to be uniformly applied to the entirety in either case ofthe pressing die having a convex shape or the pressing die having aconcave shape. Hence, because of favorable settlement of the preform,occurrence of defective molding articles may be reduced.

As described above, matching of the shape of the leading end of thepressing die used in the following molding step with the concave face,or with the curved face in the vicinity of approximately the centralposition in top view of the convex face of the preform is important. Inthe case in which the shape of the leading end of the pressing die is ashape of an approximately spherical face, it is preferred that thecurvature radius of the concave face, or of the curved face in thevicinity of approximately the central position of the convex face of thepreform be nearly equal to that of the shape of the leading end of thepressing die.

When the curvature radius of the concave face of the preform isrelatively too great, too great pressure may be applied to approximatelythe central position of the concave face of the preform. When thecurvature radius is relatively too small, the pressure may not bedirectly applied to approximately the central position of the concaveface of the preform but may be applied circularly to its periphery, andin addition, just a slight inclination of the pressing mold may lead todeviation of the pressure applied to a part of the ring. Curvatureradius of the curved face in the vicinity of approximately the centralposition of the concave face of the preform is preferably somewhatgreater than that of the shape of the leading end of the pressing diebecause it is expected that the pressure is likely to be applied moreuniformly.

When the curvature radius of the convex face of the preform isrelatively too great, the pressure may not be directly applied toapproximately the central position of the convex face of the preform butmay be applied circularly to its periphery, and in addition, just aslight inclination of the pressing die may lead to deviation of thepressure applied to a part of the ring. When the curvature radius isrelatively too small, too great pressure may be applied to approximatelythe central position of the convex face. Curvature radius of the curvedface in the vicinity of approximately the central position of the convexface of the preform is preferably somewhat smaller than that of theshape of the leading end of the pressing die because it is expected thatthe pressure is likely to be applied more uniformly.

Additionally, it is preferred that the ratio of the wall thickness ofthe preform in the vicinity of approximately the central position of theconcave face of the preform to the aforementioned diameter be 1.0 orless. Also, this ratio is more preferably 0.2 or greater. Moreover, thisratio is more preferably 0.9 or less. Accordingly, appropriate amount ofdeformation may be imparted to the preform to allow products to bemolded.

When the top surface and the under surface of the preform both have ashape including a convex face, it is preferred that the ratio of wallthickness of the preform in the vicinity of approximately the centralposition of the convex face of the top surface and the under surface tothe aforementioned diameter be 0.45 or less. Also, this ratio is morepreferably 0.1 or greater. Moreover, this ratio is preferably 0.3 orless. When the ratio falls within this range, occurrence of chipping ofthe manufactured preform can be reduced, and in addition, pressing timecan be shortened due to a small amount of deformation.

More specifically, the following is provided.

(1) A preform for molding an optical glass element which exhibits analmost circular shape having a predetermined diameter in top view andexhibits a flattened semicircular shape having a downwardly convex curveand a nearly horizontal straight line on the upper end side in side viewsuch that the distance from the lowermost part of the convex curve tothe straight line becomes a predetermined height of the flattenedsemicircular shape, and which has a concave face on the top surface anda convex face on the bottom face, wherein the concave face on the topsurface forms the lowermost part of the concave face at an approximatelythe central position in almost circular shape in the top view; theconvex face on the bottom face forms the lowermost part of the convexface at the central position of the bottom face corresponding to theaforementioned approximately the central position; and the ratio of theheight of the flattened semicircular shape to the diameter of the almostcircular shape in the top view is from 0.2 to 0.9.

(2) The preform for molding an optical glass element according to theabove aspect (1) wherein the concave face has, at approximately thecentral position in almost circular shape in the top view, ratio of thedistance of from the lowermost part of the concave face to the straightline of the upper end side in the side view to the diameter of thealmost circular shape in the top view is from 0.02 to 0.9.

(3) The preform for molding an optical glass element according to theabove aspect (1) or (2) wherein, at approximately the central positionin almost circular shape in the top view, ratio of the wall thickness ofthe preform that is a distance of from the lowermost part of the convexface to the lowermost part of the concave face in the side view to thediameter of the almost circular shape in the top view is from 0.2 to0.9.

(4) The preform for molding an optical glass element according to anyone the above aspects (1) to (3) wherein the ratio of the predeterminedheight of the flattened semicircular shape in the side view to thediameter of the almost circular shape in the top view is from 0.2 to0.9.

(5) The preform for molding an optical glass element according to anyone the above items (1) to (4) wherein the convex face has a ratio ofthe curvature radius in the vicinity of the lowermost part of the convexface to the curvature radius in the vicinity of the lowermost part ofthe concave face on the concave face being from 0.4 to 10.

(6) A preform for molding an optical glass element which exhibits analmost circular shape having a predetermined diameter in top view, andwhich has a concave face on both of the top surface and the undersurface, wherein the concave face on the top surface forms the lowermostpart of the concave face at approximately the central position in almostcircular shape in the top view; the concave face on the bottom faceforms the uppermost part of the concave face at the central position ofthe under surface corresponding to the aforementioned approximately thecentral position; and ratio of the height of the flattened ellipticshape in the side view to the diameter (D) of the almost circular shapein the top view is from 0.1 to 0.9.

(7) A preform which exhibits an almost circular shape having apredetermined diameter in top view and has a flattened elliptic shapehaving a nearly horizontal straight line on the top end and lower endsides in side view such that the distance between the top end and thelower end in the side view becomes a predetermined height (h) of theflattened elliptic shape, and which has a concave face on both of thetop surface and the under surface, wherein the concave face on the topsurface forms the lowermost part of the concave face at an approximatelythe central position in almost circular shape in the top view; theconcave face on the bottom face forms the uppermost part of the concaveface at the central position of the under surface corresponding to theapproximately central position; and ratio of the height of the flattenedelliptic shape in the side view to the diameter (D) of the almostcircular shape in the top view is from 0.1 to 0.9.

(8) The preform for molding an optical glass element according to theabove aspect (6) wherein ratio of sum (Δh1+Δh2) of a distance (Δh1) offrom the lowermost part of the concave face to the upper end side on oneconcave face in the side view and a distance (Δh2) of from the uppermostpart of the concave face to the lower end side on another concave facein the side view to the diameter (D) of the almost circular shape in thetop view is from 0.02 to 0.9.

(9) The preform for molding an optical glass element according to theabove aspect (6) or (8) wherein ratio of wall thickness (t) of thepreform that is a distance of from the lowermost part of the concaveface on one concave face to the uppermost part of the concave face onanother concave face in the side view to the diameter (D) of the almostcircular shape in the top view is from 0.2 to 0.9.

(10) The preform for molding an optical glass element according to anyone of the above aspects (6) to (9) wherein ratio of a curvature radius(R1) in the vicinity of the lowermost part of the concave face on oneconcave face in the side view to a curvature radius (R2) in the vicinityof the uppermost part of the concave face on another concave face in theside view is from 0.1 to 10.

(11) A preform for molding an optical glass element which exhibits analmost circular shape having a predetermined diameter in top view, andwhich has a convex face on both of the top surface and the undersurface, wherein the convex face on the top surface forms the uppermostpart of the convex face at an approximately the central position inalmost circular shape in the top view; the convex face on the bottomface forms the undermost part of the convex face at the central positionof the under surface corresponding to the approximately centralposition; and ratio of the wall thickness (t) of the preform that is adistance of from the uppermost part of the convex face in the top faceto the lowermost part of the convex face on the under surface in theside view to the diameter (D) of the almost circular shape in the topview is 0.45 or less.

(12) The preform for molding an optical glass element according toanyone of the above aspects (1) to (11) which comprises a silica-boricacid based or lanthanum-based optical glass that is a material preferredfor optical use.

(13) An optical element produced by subjecting the preform according toany one of the above aspects (1) to (12) to precise press molding.

According to the first preform of the present invention, a desired shapeis preferably molded by compression through pressing with an upper moldhalf having a curvature radius matched predominantly at the leading endto the curvature radius of the aforementioned concave face while theconcave face to be its top surface setting upward, against a face thatmatches the convex face on the bottom face of the preform, being a facethat has a concave shape of a lower mold half as a receiving mold half.Moreover, according to the second preform, a desired shape is preferablyobtained by compression through pressing with an upper mold half havinga curvature radius matched predominantly at the leading end to thecurvature radius of the concave face while the concave face settingupward, against a face that matches the concave face on the bottom faceof the preform, being a face that has a convex shape of a lower moldhalf as a receiving mold half. Also, in connection with the thirdpreform, a desired shape is preferably obtained by compression throughpressing with an upper mold half having a curvature radius matchedpredominantly at the leading end to the curvature radius of the convexface while the convex face setting upward, against a face that matchesthe convex face on the bottom face of the preform, being a face that hasa concave shape of a lower mold half as a receiving mold half.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross sectional view illustrating a preform.

FIG. 2 shows an explanatory view illustrating a feeding step of a moltenglass.

FIG. 3 shows an explanatory view illustrating a step of manufacturing apreform by pressing a glass block with an upper mold half.

FIG. 4 shows a view illustrating a state after removing the upper moldhalf from the state shown in FIG. 3.

FIG. 5 shows a cross sectional view illustrating a preform according tothe second embodiment.

FIG. 6 shows an explanatory view illustrating a feeding step of moltenglass in the manufacturing step according to the second embodiment.

FIG. 7 shows an explanatory view illustrating a step of manufacturing apreform by pressing a glass block with an upper mold half in themanufacturing step according to the second embodiment.

FIG. 8 shows a view illustrating a state after removing the upper moldhalf from the state shown in FIG. 7.

FIG. 9 shows a cross sectional view illustrating a preform according tothe third embodiment.

FIG. 10 shows an explanatory view illustrating a feeding step of amolten glass in the manufacturing step according to the thirdembodiment.

FIG. 11 shows an explanatory view illustrating a step of manufacturing apreform by pressing a glass block with an upper mold half in themanufacturing step according to the third embodiment.

FIG. 12 shows a view illustrating a state after removing the upper moldhalf from the state shown in FIG. 11.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the first preform of the present invention will beexplained in detail with reference to FIGS. 1 to 4. Furthermore, thesecond preform of the present invention will be explained in detail withreference to FIGS. 5 to 8. Moreover, the third preform of the presentinvention will be explained in detail with reference to FIGS. 9 to 12.In the following description of the embodiments, common constitutiveelements are designated by the same reference number, and theexplanation thereof will be omitted or simplified.

Height of the preform referred to herein means a distance to the highestposition in the vertically upward direction when the preform washorizontally immobilized such that one side faces downward. Center wallthickness of the preform means the length of the thickness of thepreform at approximately the center of the preform. Depth of the preformat the concave part means a distance to a straight line which was drawnto be nearly horizontal on the upper end side or bottom part side fromthe low point of the concave part curvature in side view, when thepreform was horizontally immobilized such that one side faces downward.External diameter of the preform means a diameter of the preform viewedfrom above.

First Preform

FIG. 1 shows a cross sectional view of the first preform (or glass gob)10 of the present invention. The preform 10 has a circular shape havinga diameter (D) in top view, or a shape somewhat flattened from ahemispherical shape having a diameter (D) viewed sterically. Umbilicalposition of this hemispherical shape is an under surface central point12 of the leading end part of the convex shape shown at the bottom sideof the Figure, which corresponds to the lowermost part of the convexface. Also in bottom view, the preform exhibits a circular shape havinga diameter (D) with this under surface central point 12 as a center. Across section 19 having a boomerang-like shape (or opened V shape) towhich oblique lines are added on this under surface central point 12 ispresented. This preform shows a rotation symmetry with respect to anaxis perpendicularly extending in the Figure and passing the undersurface central point 12, and has a shape that is in plane symmetry withrespect to a vertical plane including the cross section 19.

The cross section 19 has a shape extending obliquely upward with respectto both right and left as described above, and the right and leftleading ends in the cross section follow a circular arc 14 having apredetermined radius, and reach to the contour line of the cross sectionsmoothly connecting to the top surface of the preform 10. This topsurface has a concave face 16 gently descending from the circumferentialpart 22 having the maximum height of this preform in the Figure (whichwill be explained below), and the contour line in the cross section 19passes over the inflection point 17 through the aforementioned circulararc 14 to be directed toward the center of the recess of the concaveface 16. The curvature changes in the vicinity of the inflection point17, but position and extent of the inflection may vary depending on themanufacturing method and manufacturing condition upon manufacture ineffect.

Vicinity of the under surface central point 12 of the leading end of theconvex part of the preform 10 consists of a part of a spherical surface“sphere R1”. Center 18 of the “sphere R1” is situated on the axis of theaforementioned rotation symmetry but is situated higher than the surfaceof the concave face 16. Scope of this “sphere R1” falls within the scopeto comply with “Deg 1” or less in the cross section 19. Similarly, thecentral part of the concave face 16 of the preform 10 consists of a partof the spherical surface of a “sphere R2”. Center 20 of the “sphere R2”is situated on the axis of the aforementioned rotation symmetry. Thescope of this “sphere R2” falls within the scope to comply with “Deg 2”or less in the cross section 19.

A region between the under surface central point 12 and the circular arc14 of the leading end of the convex part of the preform 10 has apredetermined spherical surface; however, the center of the same is notsituated on the axis of the circular symmetry, but is shifted to theright in the Figure (to the left in symmetry of the convex face on theright side).

The parameters D, R1, R2, Deg 1 and Deg 2 which may be used herein mayfall within the following ranges, respectively. TABLE 1 Lower limitUpper limit D 5.0 20 R1 8 60 R2 6 20 Deg1 3 30 Deg2 10 40

Height of the preform referred to herein may mean, for example, when thepreform 10 was horizontally immobilized such that its under surfacecentral point 12 which shall be top of the convex part of the preformfaces downward, a distance of from the face where the preform is placedto the highest position in the vertically upward direction,specifically, means the distance (h) in FIG. 1. In other words, in theflattened semicircular shape having a downwardly convex curve and anearly horizontal straight line on the upper end side in side view, thedistance of from the lowermost part of the convex curve to the straightline is the predetermined height (h) of the flattened semicircularshape.

Depth of the preform at the concave part corresponds to, for example,when the preform was horizontally immobilized such that the convex partof the preform faces downward, difference between the vertical distance(t) from the face where the preform is placed to the bottom point of thepreform concave part curvature, and the aforementioned height (h) of thepreform. Specifically, the depth corresponds to (Δh) in FIG. 1. In otherwords, it corresponds to a distance (Δh) from the lowermost part of theconcave face on the top surface to the aforementioned straight line ofthe aforementioned upper end side, at approximately the central positionin almost circular shape in top view.

Center wall thickness of the preform means a distance of from the toppoint of the convex part curvature to the bottom point of the concavepart curvature. In FIG. 1, the center wall thickness corresponds to thedistance represented by (t) in FIG. 1.

External diameter of the preform herein means a diameter of the preformviewed from above. In FIG. 1, the external diameter means the distancerepresented by the diameter (D).

In this Example, the depth (Δh) of the concave part is preferably 0.02times or greater of the external diameter of the preform. Also, it ispreferably 0.9 times or less of the depth (Δh) of the concave part. Whenthe recession of the concave part on the concave face 16 is too greatwith respect to the external diameter, the preform is liable to bechipped when the preform is molded, thereby leading to possibility ofincrease in percent of defective articles produced. In contrast, whenthe recession is too small, the amount of deformation of the glass inthe following precise press molding becomes too great, thereby leadingto possibility of reduction in lifetime of the mold and increase in thecycle time in precise press molding.

In Examples of the present invention, it is preferred that the centerwall thickness (t) be 0.2 times or greater of the external diameter (D)of the preform. Also, it is preferably 0.9 times or less of the externaldiameter (D) of the preform. When the center wall thickness is too smallwith respect to the external diameter, the preform is liable to bechipped when it is molded, thereby leading to possibility of increase inpercent of defective articles produced. In contrast, when the centerwall thickness is too great, the amount of deformation of the glass inthe following precise press molding becomes so great that reduction inlifetime of the mold and increase in the cycle time may be caused.

In Examples of the present invention, it is preferred that the height(h) have the lower limit of 0.2 times and the upper limit of 0.9 timesof the external diameter of the preform. When the height herein is toosmall with respect to the external diameter, excess pad will spread toincrease the external diameter, thereby yielding a thin wall which isliable to be chipped. In contrast, when the height is too great, amountof deformation in the molding becomes so great that reduction inlifetime of the mold and increase in the cycle time may occur.

The external diameter and the center wall thickness of the preform ofthe present invention are not particularly limited, but in light of theworking efficiency, the upper limit of the external diameter may bepreferably 20 mm, more preferably 15 mm, and most preferably 12 mm.Furthermore, the lower limit of the external diameter may be preferably5 mm, more preferably 7 mm, and most preferably 9 mm. Meanwhile, theupper limit of the center wall thickness may be preferably 18 mm, morepreferably 12 mm, and most preferably 8.5 mm. Moreover, the lower limitof this center wall thickness may be preferably 1.0 mm, more preferably2.0 mm, and most preferably 3.5 mm.

For the measurement of the curvature radius R of the preform of thepresent invention, for example, it may be carried out using a ψ4measuring instrument. Specifically, the ψ4 measuring instrument is a cupgauge including a cup having a internal diameter of “ψ4” and a sensingpin. Amount of displacement of the sensing pin can be read with adigital meter. First, the position of the top surface of the cup andposition of the sensing pin are aligned using a base plate, and theposition is defined as a base (reading on the digital meter being zero).Next, the measuring face of the preform of which is pressed against thecup, and the sensing position is read from the digital meter (ΔH). Thisvalue is substituted for the following formula (I) to derive thecurvature radius R. $\begin{matrix}{R = \frac{4^{2} + {4 \times \left( {\Delta\quad H} \right)^{2}}}{8 \times \Delta\quad H}} & (1)\end{matrix}$

The first preform of the present invention can be manufactured by thefollowing manufacturing steps schematically illustrated in FIGS. 2 to 4.FIG. 2 shows an apparatus 50 and the step for feeding a molten glass toaccumulate a glass block 11 having a predetermined weight. With respectto the glass block 11 having a predetermined weight prior to molding, asshown by a molten glass 56 dropping in part at the feed port from anozzle 52 of the apparatus for feeding a molten glass, a molten glassheated to a predetermined temperature is fed and accumulated in a lowermold half (or receiving mold) 54 in a predetermined amount. The lowermold half 54 herein has fine wind holes opened, thus as shown by anarrow 62, any of hot wind, warm wind and cold wind can be fed thereto.This feeding serves to prolong the lifetime of the lower mold half, butthe wind may not also be fed. The molten glass accumulated in the lowermold half 54 forms a glass block 11 while being cooled to some extent bythe gas atmosphere, and moves to a place where the upper mold half 72 isapplied together with the lower mold half 54.

FIG. 3 shows an apparatus 70 and a step for forming the concave part ofthe preform by pressing force with the upper mold half 72. The glassblock 11 moved together with the lower mold half 54 turns into the stateof a predetermined glass surface temperature, pressed by the upper moldhalf 72 disposed at an opposing position. Accordingly, a preform for anoptical element 13 having a concave part can be obtained. During thisprocess, the wind having a predetermined temperature is fed from theface 60 of the lower mold half 54 as shown by the arrow 62, andsimultaneously, the wind having a predetermined temperature is also fedfrom the upper mold half 72 as shown by an arrow 76.

Moreover, the shape of the lower mold half 54 is not limited by aparticular curvature radius, and it may or may not emit a gas itself.FIG. 4 shows a status of taking off the upper mold half 72 that formsthe concave part on the concave face 16 shown in FIG. 3 to remove thepreform 15 as a product. In the description of FIGS. 2 to 4, a lenshaving a concavo-convex shape or the like is assumed as an opticalelement manufactured with the preform. However, when the finallyobtained molded product is a biconcave lens or the like, lower side facein the Figure can be formed to have a concave shape by using thereceiving mold of the preform with a convex shape or by moving the pieceonce received on a concave shape to the receiving mold with a convexshape. Hence, the entirety or apart of the formed face can be freelyregulated to have the concave shape according to the method as describedabove.

Second Preform

FIG. 5 shows a cross sectional view illustrating one example of thesecond preform of the present invention. This preform 10 a has acircular shape having a diameter (D) in top view, or a shape includingflattened portions of a hemispherical shape having a diameter (D) andbeing connected in a vertically symmetric manner viewed sterically. Thatis, the second preform is different from the first preform shown in FIG.1 in terms of the under surface forming a concave part as shown in FIG.5.

The cross section has a shape extending obliquely upward with respect toboth right and left and further extending obliquely downward withrespect to both right and left, and the right and left leading ends inthe cross section follow a circular arc having a predetermined radiusand reach to the contour line smoothly connecting to the top surface andthe under surface of the preform. These top and under surfaces have aconcave face gently sloping from the circumferential part. Curvatureradius and depth of this concave face may be altered freely depending onthe manufacturing condition. Parameters D, R1, R2, Deg 1 and Deg 2 whichmay be used herein may fall within the following range, respectively.TABLE 2 Lower limit Upper limit D 5.0 20 R1 6 60 R2 6 60 Deg1 3 40 Deg23 40

In this embodiment, sum (Δh1+Δh2) of depths of both of the concave partsis preferably 0.02 times or greater of the external diameter (D), morepreferably 0.05 times or greater, and most preferably 0.1 times orgreater. Furthermore, sum (Δh1+Δh2) of depths of both of the concaveparts is preferably 0.9 time or less, more preferably 0.7 times or less,and most preferably 0.5 times or less. When the depths of both of theconcave parts are too great with respect to the external diameter,chipping is liable to be caused when the preform is molded, therebyleading to increase in percent of defective articles produced. Incontrast, when the depths are too small, the amount of deformation ofthe glass in the following precise press molding becomes too great,thereby leading to possibility of reduction in lifetime of the mold andincrease in the cycle time in precise press molding.

In this embodiment, the center wall thickness (t) is preferably 0.2times or greater of the external diameter (D) of the preform, morepreferably 0.3 times or greater, and most preferably 0.4 times orgreater. Also, it is preferably 0.9 times or less of the externaldiameter D of the preform, more preferably 0.8 times or less, and mostpreferably 0.7 times or less. When the center wall thickness is toosmall with respect to the external diameter, the preform is liable to bechipped when the preform is molded, thereby leading to possibility ofincrease in percent of defective articles produce. In contrast, when thecenter wall thickness is too great, the amount of deformation of theglass in the following precise press molding becomes too great, therebyleading to possibility of reduction in lifetime of the mold and increasein the cycle time in precise press molding.

In this embodiment, the height h has the lower limit of 0.1 times, morepreferably 0.2 times and most preferably 0.3 times of the externaldiameter D of the preform, and has the upper limit of 0.9 times, morepreferably 0.8 times and most preferably 0.7 times. When the heightherein is too small with respect to the external diameter, excess padwill spread to increase the external diameter, thereby yielding a thinwall which is liable to be chipped. In contrast, when the height is toogreat, amount of deformation in the molding becomes so great thatreduction in lifetime of the mold and increase in the cycle time may becaused.

The external diameter and the center wall thickness of the preform ofthe present invention are not particularly limited, but in light of theworking efficiency, upper limit of the external diameter may bepreferably 20 mm, more preferably 15 mm, and most preferably 12 mm.Furthermore, lower limit of the external diameter may be preferably 5mm, more preferably 7 mm, and most preferably 9 mm. Meanwhile, upperlimit of the center wall thickness may be preferably 18 mm, morepreferably 12 mm, and most preferably 8.4 mm. Moreover, lower limit ofthis center wall thickness may be preferably 1.0 mm, more preferably 2.0mm, and most preferably 3.5 mm.

Manufacturing Example

The second preform in this embodiment can be manufactured by themanufacturing steps similar to those for the first preform. Morespecifically, as shown in FIGS. 6 to 8, the difference from the firstpreform shown in from FIG. 2 to FIG. 4 is found only in terms of theface 60 a on the lower mold half 54 a being convex. Other matters, i.e.,apparatus for and step of feeding of the molten glass and accumulatingthe glass block having a predetermined weight (FIG. 6), step of pressingwith the upper mold half 72 to manufacture the preform (FIG. 7), step oftaking off the upper mold half and removing the product (FIG. 8) may beemployed which are similar to those in the method for the first preform.Accordingly, preforms 13 a and 15 a having a biconcave part can bemanufactured.

Third Preform

FIG. 9 shows a cross sectional view illustrating one example of thethird preform of the present invention. This preform 10 b has a circularshape having a diameter (D) in top view, or a shape including flattenedportions of a hemispherical shape having a diameter (D) and beingconnected in a vertically symmetric manner viewed sterically. That is,the third preform is different from the first preform shown in FIG. 1 interms of the upper surface forming a convex part as shown in FIG. 9.

The cross section has a shape extending obliquely upward with respect toboth right and left, and the right and left leading ends in the crosssection follow a circular arc having a predetermined radius to reach tothe contour line smoothly connecting to the top surface of the preform.These top and under surfaces have a convex face gently curved from thecircumferential part. Diameter (D) and wall thickness (t) of this thirdpreform can be altered freely depending on manufacturing condition s.Parameters D and t which may be used herein may fall within thefollowing ranges, respectively. TABLE 3 Lower limit Upper limit D 10 20t 3 6

In this embodiment, the center wall thickness t is preferably 0.45 timesor less of the external diameter D of the preform, and more preferably0.40 times or less. In particular, when a glass having a high viscosityor a glass having high Tg is used, the center wall thickness of 0.3times or less of the external diameter D of the preform is mostpractical in light of less charging amount and reduction in energy foruse. When the center wall thickness is too great with respect to theexternal diameter, press molding requires a long time, which may resultin economic disadvantages due to the need for high levels of greatenergy consumption. Additionally, the wall thickness t of the preform ispreferably 0.05 times or greater of the external diameter D, and morepreferably 0.1 times or greater, because too small a wall thickness ofthe preform may lead to increased probability of chipping in molding.

The external diameter and the center wall thickness of the preform ofthe present invention are not particularly limited, but in light of theworking efficiency, the upper limit of the external diameter may bepreferably 20 mm, more preferably 18 mm, and most preferably 17 mm.Furthermore, the lower limit of the external diameter may be preferably10 mm, more preferably 12 mm, and most preferably 13 mm. In addition,the upper limit of the center wall thickness may be preferably 6 mm,more preferably 5.5 mm, and most preferably 5 mm. Moreover, the lowerlimit of the center wall thickness may be preferably 3 mm, morepreferably 3.5 mm, and most preferably 4 mm.

Manufacturing Example

The third preform in this embodiment can be manufactured by themanufacturing steps similar to those for the first preform. Morespecifically, as shown in FIGS. 10 to 12, difference from the firstpreform shown in FIG. 2 to FIG. 4 is found only in terms of the face 74b on the upper mold half 72 b being concave. Other matters, i.e.,apparatus for and step of feeding of the molten glass and accumulatingthe glass block having a predetermined weight (FIG. 10), step ofpressing with the upper mold half 72 b to manufacture the preform (FIG.11), step of taking off the upper mold half and removing the product(FIG. 12) may be employed which are similar to those in the method forthe first preform. Accordingly, preforms 13 b and 15 b having a biconvexpart can be manufactured.

EXAMPLES

Next, specific examples will be explained.

Example 1

Melting was conducted in a glass melting furnace to give the glassmelting temperature of 1100 to 1300° C., while a nozzle 52 at theleading end of a pipe was heated at a temperature of 900 to 1200° C.Next, a lower mold half 54 was disposed immediately below the nozzle 52,and a glass block 11 was dropped, or elevated to approach the nozzle 52.When the concave molding face of the lower mold half 54 was filled withthe molten glass accordingly, the lower mold half 54 was lowered. Themolten glass was cut then to obtain a glass block 11.

The material of the lower mold half 54 was a porous metal, and emits anyone of inert gases such as air, oxygen, nitrogen, argon and the like orany mixed gas thereof at 0.5 to 10 L/min. The lower mold half 54 holdingthe glass block 11 was pressed with the upper mold half 72 to obtain apreform for an optical element 13 having a concave part. In thisexample, two kinds, i.e., a silica-boric acid based or lanthanum-basedoptical glass were used as an optical glass composition.

Example 2

A glass block 11 was obtained with a lower mold half 54 with no emissionof a gas under a temperature condition that is similar to Example 1. Thelower mold half 54 holding this glass block 11 was moved immediatelybelow an upper mold half 72 provided with fine pores that emit the gas,and simultaneously, was pressed by a pressing face 74 of the upper moldhalf 72 disposed at an opposing position under a condition of the glasssurface temperature of 800 to 1150° C. Thus, a preform for an opticalelement 13 having a concave part was obtained.

In the pressing method, pressing against an open face of the glass block11 was serially conducted for 1 to 10 sec. Surface of the preform for anoptical element 13 having a concave part obtained according to thismethod was a smooth mirror face.

The glass preform produced in this manner had a center wall thicknessbeing 0.3 times of the external diameter, a depth of the concave partbeing 0.06 times of the external diameter, and a height being 0.4 timesof the external diameter.

Example 3

A preform for an optical element having a biconcave part was obtainedthrough manufacturing according to a similar method to Example 1 exceptthat the lower mold half in Example 1 was changed to a lower mold halfhaving a convex shape. In this Example, two kinds, i.e., a silica-boricacid based or lanthanum-based optical glass were used as an opticalglass composition.

Example 4

A preform for an optical element having a biconcave part was obtainedthrough manufacturing according to a similar method to Example 2 exceptthat the lower mold half in Example 2 was changed to a lower mold halfhaving a convex shape. In the pressing method, pressing against an openface of the glass block was serially conducted for 1 to 10 sec. Surfaceof the preform for an optical element having a biconcave part obtainedaccording to this method was a smooth mirror face. Also, thus resultingpreform had a center wall thickness 0.4 times the external diameter, thesum of depths of the biconcave parts depth 0.2 times the externaldiameter, and the height 0.6 times the external diameter.

Example 5

A preform for an optical element having a biconvex part was obtainedthrough manufacturing according to a similar method to Example 1 exceptthat the upper mold half in Example 1 was changed to an upper mold halfhaving a concave part. In this Example, two kinds, i.e., a silica-boricacid based or lanthanum-based optical glass were used as an opticalglass composition.

Example 6

A preform was manufactured according to a similar method to Example 5 toyield different physical properties. Physical property values of Example5 and Example 6 are shown in Table 4. TABLE 4 Example 5 Example 6 D 13.514.0 T 5.0 6.0 t/D 0.37 0.43 R1 (free surface, top surface) 42 64 R2(mold surface, under surface) 42 30 R1/R2 1.0 2.1

Test Example 1

Using the preform for an optical element produced in Example 1 or 2, aprecise press test was carried out. The preform of the present inventionused in Test Example 1 had a center wall thickness 0.3 times theexternal diameter, a depth of the concave part 0.06 times the externaldiameter, and a height 0.4 times the external diameter.

The preform for an optical element was heated to a temperaturecorresponding to 10⁴ to 10¹¹ Pa·s. Next, the preform was compressed withthe upper mold half at 100 to 300 kg/cm², for 3 to 60 sec, followed bycooling and mold release to obtain a final molded product.

Moreover, as a Comparative Example, a precise press test was carried outon a biconvex preform having an external diameter of 10 mm and a centerwall thickness of 6.5 mm under the same pressing condition. The presentpreform for an optical element exhibited stable settlement in precisepress molding, and very few defective molded articles were produced.

Comparison of defect extent of the present preform for an opticalelement and, for example, the biconvex glass gob is shown in Table 5.The defect extent herein means chips, cracks, fusion with the mold andthe like. For example, percent defect extent of 2% means the ratio ofthe number of occurrence of defects per number of press shots. TABLE 5Preform of the present Comparative biconvex Number of manufacturingmethod glass gob percent press shots percent defective extent defectiveextent 100 shots 0% 0% 200 shots 0% 2% 500 shots 0% 3%

As shown in Table 5, the preform for an optical element of the presentinvention generated no defectives in serial 500 shots, while thebiconvex gob generated defectives in 200 shots. This is believed to becaused by the biconvex glass gob which exhibits unstable settlement inpressing and which involves great amount of deformation.

Test Example 2

Using the preform for an optical element produced in Example 3 or 4, aprecise press test was carried out. The preform of the present inventionused in Test Example 2 had a center wall thickness 0.4 times theexternal diameter, the sum of depths of both concave parts 0.2 times theexternal diameter, and a height 0.6 times the external diameter.

The preform for an optical element was heated to a temperaturecorresponding to 10⁴ to 10¹¹ Pa·s. Next, the preform was compressed withthe upper mold half at 100 to 300 kg/cm², for 3 to 60 sec, followed bycooling and mold release to obtain a final product.

Moreover, as a Comparative Examples, a precise press test was carriedout on a biconvex preform having an external diameter of 10 mm and acenter wall thickness of 6.5 mm (Comparison 2), and on a convexo-concavepreform having an external diameter of 11 mm and a center wall thicknessof 5.0 mm (Comparison 1) under the same condition. The present preformfor an optical element exhibited extremely stable settlement in precisepress molding, and very few defective molded articles were produced.

Comparison of defect extent of the present preform for an opticalelement and, for example, the biconvex preform and the convexo-concavepreform is shown in Table 6. The defect extent herein means chips,cracks, fusion with the mold and the like. For example, percent defectof 2% means the ratio of the number of occurrence of defects per numberof press shots. TABLE 6 Preform of the Concavo-convex Biconvex presentpreform of preform of Number of invention Comparison 1 Comparison 2press shots percent defects percent defects percent defects 100 shots 0%0% 0% 500 shots 0% 0% 3% 1000 shots  0% 2% 6%

As shown in Table 6, the preform for an optical element of the presentinvention generated no defectives in serial 1000 shots, while thebiconvex preform already generated 3% defectives in 500 shots, and theconvexo-concave preform generated defectives in 1000 shots. This isbelieved to be principally caused due to settlement of the preform inpressing.

As described hereinabove, the present invention is characterized by apreform for an optical element which is produced by receiving a moltenglass heated to a predetermined temperature on a receiving mold and thenpressing with an upper mold half disposed at an opposing position, andinvolves a specified relationship of the external diameter, depth, andheight.

Use of the preform involving the specified relationship of the externaldiameter, depth, and height enables decrease in chips and cracks inprecise press molding, and reduction of percent of generation ofdefective articles. In addition, because the amount of charge by themold may be decreased, less deformation of the glass is achieved, andthe lifetime of the mold can be extended. Accordingly, frequency ofrepairing the pressing mold may be reduced, and thus, an inexpensivepreform for an optical element having a concave shape with favorableappearance and quality can be provided. Furthermore, because of a formedrecession having a concave shape, settlement of the glass gob issatisfactory in mold press molding and less amount of deformation isattained, thereby leading to reduction in costs due to shortening of thepress time period.

While preferred embodiments of the present invention have been describedand illustrated above, it is to be understood that they are exemplary ofthe invention and are not to be considered to be limiting. Additions,omissions, substitutions, and other modifications can be made theretowithout departing from the spirit or scope of the present invention.Accordingly, the invention is not to be considered to be limited by theforegoing description and is only limited by the scope of the appendedclaims.

1-11. (canceled)
 12. A preform for molding an optical glass element which exhibits an almost circular shape having a predetermined diameter in top view and exhibits a flattened semicircular shape having a downwardly convex curve and a nearly horizontal straight line on the upper end side in side view such that the distance from the lowermost part of the convex curve to the straight line becomes a predetermined height of the flattened semicircular shape, and which has a concave face on the top surface and a convex face on the bottom face, wherein the concave face on the top surface forms the lowermost part of the concave face at an approximately the central position in almost circular shape in said top view; the convex face on the bottom face forms the lowermost part of the convex face at the central position of the bottom face corresponding to said approximately the central position; and the ratio of said height of said flattened semicircular shape to said diameter of the almost circular shape in said top view is from 0.2 to 0.9.
 13. The preform for molding an optical glass element according to claim 12 wherein said concave face has, at approximately the central position in almost circular shape in said top view, ratio of the distance of from said lowermost part of the concave face to said straight line of said upper end side in said side view to said diameter of the almost circular shape in said top view is from 0.02 to 0.9.
 14. The preform for molding an optical glass element according to claim 12 wherein, at approximately the central position in almost circular shape in said top view, ratio of the wall thickness of the preform that is a distance of from said lowermost part of the convex face to said lowermost part of the concave face in said side view to said diameter of the almost circular shape in said top view is from 0.2 to 0.9.
 15. The preform for molding an optical glass element according to claim 12 wherein said convex face has a ratio of the curvature radius in the vicinity of said lowermost part of the convex face to the curvature radius in the vicinity of said lowermost part of the concave face on said concave face being from 0.4 to
 10. 16. A preform for molding an optical glass element which exhibits an almost circular shape having a predetermined diameter in top view, and which has a concave face on both of the top surface and the under surface, wherein the concave face on the top surface forms the lowermost part of the concave face at approximately the central position in almost circular shape in said top view; the concave face on the bottom face forms the uppermost part of the concave face at the central position of the under surface corresponding to said approximately the central position; and ratio of the height of the flattened elliptic shape in the side view to the diameter of the almost circular shape in the top view is from 0.1 to 0.9.
 17. The preform for molding an optical glass element according to claim 16 wherein ratio of sum of a distance of from said lowermost part of the concave face to the upper end side on one concave face in said side view and a distance of from the uppermost part of the concave face to the lower end side on another concave face in the side view to the diameter of the almost circular shape in said top view is from 0.02 to 0.9.
 18. The preform for molding an optical glass element according to claim 16 wherein ratio of wall thickness of the preform that is a distance of from said lowermost part of the concave face on one concave face to said uppermost part of the concave face on another concave face in said side view to the diameter of the almost circular shape in said top view is from 0.2 to 0.9.
 19. The preform for molding an optical glass element according to claim 16 wherein ratio of a curvature radius in the vicinity of said lowermost part of the concave face on one concave face in said side view to a curvature radius in the vicinity of said uppermost part of the concave face on another concave face in said side view is from 0.1 to
 10. 20. An optical element produced by subjecting the preform according to claim 12 to precise press molding.
 21. An optical element produced by subjecting the preform according to claim 16 to precise press molding.
 22. A preform for molding an optical glass element which exhibits an almost circular shape having a predetermined diameter in top view, and which has a convex face on both of the top surface and the under surface, wherein the convex face on the top surface forms the uppermost part of the convex face at an approximately the central position in almost circular shape in said top view; the convex face on the bottom face forms the undermost part of the convex face at the central position of the under surface corresponding to said approximately the central position; and ratio of the wall thickness of the preform that is a distance of from the uppermost part of the convex face on the top face to the lowermost part of the convex face on the under surface in said side view to the diameter of the almost circular shape in the top view is 0.45 or less.
 23. An optical element produced by subjecting the preform according to claim 22 to precise press molding. 